stacks.json is out of date for RHEL8-based stack images

stacks.json is out of date:

1. currently use the old java image as the basis for the EAP-based stacks, and until we switch that over, we'll be relying on the RHEL7 version of the EAP72 base image.

2. We should also remove the extra Java stack image (the RHEL7 one) from stacks.json (and anywhere else it's referenced) so that we only have the one Java8+EAP72 image in the stacks (and use that for all the middleware derivative stacks).

3. Stacks that I just moved to RHEL 8 should have their metadata updated so they don't say "RHEL7.6" but "RHEL8" (this isn't a big deal – we had 7.4 in there even though we were publishing based on 7.6 in CRW 1.1, but since i discovered it I've been trying to keep that file up to date and accurate)

4. Once stacks.json is updated, we'll need to rebuild the server image. We could also move to the 2x larger RHEL image at the same time, if PM agrees this won't be a performance problem. See CRW-262